Method and system for digital pen assembly

ABSTRACT

A receiving unit for receiving an acoustic signal from a digital pen, comprises: at least two ultrasound receivers, for receiving ultrasound signals from the digital pen; an electric circuit connected to said ultrasound receivers, and configured to extract ultrasound signals received by said ultrasound receivers, to enable tracking a position of said pen, and a biased clip for holding paper in fixed relationship to said ultrasound receivers. The clip may be metal, and alignment and location features may be provided for fitting sheets of paper for writing.

RELATED APPLICATION/S

This application is a divisional of U.S. patent application Ser. No.11/884,894 filed on Aug. 22, 2007, which is a National Phase of PCTPatent Application No. PCT/IL2006/000373 filed on Mar. 23, 2006, whichclaims priority of U.S. Provisional Patent Application No. 60/664,202filed on Mar. 23, 2005. The contents of the above applications areincorporated herein by reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to acoustic positioning methods, and moreparticularly, but not exclusively to a method and an apparatus for dataentry using an acoustic signal transmitting pen input device.

Digital writing instruments, interchangeably referred to herein asDigital Pens, regardless of whether they actually write on paper, can beused to capture pen strokes on paper and digitize them.

For example, with a digital pen, pen strokes may be converted byhandwriting recognition software to a digitally stored record of thewriting. In this way, a laborious, tedious step in modern writtencommunication, namely, the manual transcribing of handwriting into acomputerized word processor, is eliminated, greatly increasingproductivity.

Sensing a time-dependent position of the pen and converting thepositions to pen strokes may be used for input of digitalrepresentations of the pen strokes to a handwriting recognition device.

As known in the art, ultrasonic systems can be used in which a specialpen generates or alters an ultrasonic signal, as the pen is moved acrossa piece of paper.

The ultrasonic signal is sensed by receivers and correlated to aposition vis-à-vis each receiver, as the outputs of the receivers istriangulated and correlated to absolute pen positions. A sequence of penpositions can then be digitized for input into handwriting recognitionengines.

An advantage with ultrasonic systems is that the user of the ultrasonicsignal emitting device can use the device to write on an ordinary pieceof paper that is placed on or nearby a base station, which receives theultrasonic signals and converts the signals to alpha-numeric characters.

There are many currently known in the art methods for data entry usingan acoustic impulse transmitting pen input device.

U.S. Pat. No. 4,814,552, to Stefik, filed on Dec. 2, 1987, entitled“Ultrasonic position input device”, describes an input device, orstylus, for entering hand drawn forms into a computer comprising awriting instrument, a pressure switch for determining whether theinstrument is in contact with the writing surface, an acoustictransmitter for triangulating the position of the stylus on the surface,and a wireless transmitter for transmitting data and timing informationto the computer.

In operation, the stylus described by Stefik transmits an infraredsignal which the system receives immediately, and an ultrasound pulsewhich two microphones receive after a delay which is a function of thespeed of sound and the distance of the stylus from each microphone.

U.S. Pat. No. 6,654,008, to Ikeda, filed on Nov. 27, 2001, entitled“Electronic whiteboard and penholder used for the same”, describes anelectronic whiteboard capable of being drawn, using marker pens ofseveral colors, and one penholder for use in such an electronicwhiteboard.

In Ikeda's patent, an infrared light emitting unit emits infrared lightcontaining color information of the marker pen, an ultrasonic waveemitting unit emits the ultrasonic wave, and color informationchangeover means changes over color information depending on the colorof marker pen. The electronic whiteboard main body receives the infraredlight and ultrasonic wave emitted from the penholder, and issuesinformation about a position of the penholder depending on the receptiontiming of the infrared light and ultrasonic wave.

U.S. Pat. No. 6,876,356 to Zloter, filed on Mar. 18, 2002, entitled“Digitizer pen”, describes a digitizer pen system including a pen havinga means protruding from the pen's writing tip, for preventing fingersblocking communication with a base unit.

U.S. Pat. No. 6,184,873 to Ward, filed on Jan. 20, 1998, entitled “Penpositioning system”, describes a pen positioning system including a pen.The pen has multiple output elements and is adapted to accuratelydetermine the location of the pointing tip of the pen, in relation to anelectronic tablet.

The output elements, preferably ultrasonic transmitters having distinctfrequencies, are located a fixed distance from each other, and are alsorelated in space to the pointing tip of the pen.

A detection system is used to receive the output signals from the outputelements, isolate the output signals from each other, and process themindependently, to determine the location of the output elements and ofthe pointing tip of the pen.

U.S. Pat. No. 6,703,570 to Russel, filed on May 10, 2000, entitled“Digital pen using ultrasonic tracking”, describes a digital pen system.Russel's system includes an elongated pen defining a writing tip, and anultrasonic transducer oriented on the pen to direct frames of ultrasonicenergy outwardly from the pen, with each frame including plural receivepulses.

The digital pen system in Russel's patent further includes two or moredetectors positioned on a base, such as a laptop computer, for receivingthe pulses, with each pulse being associated with at least one pulsetime of arrival (TOA) relative to at least one detector. Russel's systemfurther includes a processor positioned on the base, receiving signalsfrom the detectors, and outputting position signals representative ofpositions of the pen, based on the received signals.

However, there are inherent problems in current acoustical technologyand in the implementation of the current acoustical technology indigital pens, such as the digital pens described in the patents citedhereinabove.

Among the disadvantages of current acoustic technology are: lack ofaccuracy, lack of multi-devices support, high power consumption, etc.The problems have implications on the mechanical design of existing dataentry using an acoustic impulse transmitting devices.

Apart from that, there are manufacturing problems related to theassembly of the acoustic transmitter and its incorporation in a digitalpen or the like. For instance, such problems may arise in connecting anacoustic transmitter to a flexible printed circuit board (PCB). Thereare also marketing issues, such as differentiation between products bychanging their appearance, while keeping the functional parts the same.

There is thus a widely recognized need for, and it would be highlyadvantageous to have, an apparatus or a method devoid of the abovelimitations.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided areceiving unit for receiving an acoustic signal from a digital pen,comprising:

at least two ultrasound receivers, for receiving ultrasound signals fromthe digital pen; and

an electric circuit connected to the ultrasound receivers, andconfigured to extract ultrasound signals received by the ultrasoundreceivers, to enable tracking a position of the pen, and

a biased clip for holding paper in fixed relationship to the ultrasoundreceivers.

The clip may be metal.

An embodiment may comprise a depression for accurate location of thepaper.

In an embodiment, the depression is for accurate location of a corner ofthe paper.

In an embodiment, the clip comprises a release-extension for opposingthe bias to release the clip.

In an embodiment, the extraction comprises referencing a reference modelcomprising data pertaining to expected reference signals.

In an embodiment, friction pads may be located in association with theclip to improve grip of the paper.

In an embodiment, a surface of the receiving unit opposite the clip isshaped to cause deformation of the paper held by the clip to improvegrip of the paper.

An embodiment may comprise elongated grips located to hold the paper atan oblique angle.

In an embodiment, ultrasound receivers may be electret microphones orMEMS microphones.

An embodiment may comprise an attachment device, for removably attachingthe receiving unit to another item.

In an embodiment, the at least two ultrasound receivers are positionedless than 65 mm apart from each other.

In an embodiment, the receiving unit further comprises a housing,configured to house the ultrasound receiver and having a changeablereceiving unit cover element.

An embodiment may be used with a digital pen comprising a skeleton and aremovable skin:

the skeleton holding together in fixed relationship:

-   -   an electric circuit;    -   an acoustic transmitter, detached from the electric circuit, and        configured to transmit acoustic signals; and

the removable skin comprising a pen housing fitting over the skeleton.

The pen housing and the receiving unit cover element may correspond.

According to a second aspect of the present invention there is provideda digital pen system, comprising:

-   -   a digital pen having an elongated body terminating in a writing        tip, a writing element protruding from the writing tip, and an        acoustic signal transmitter deployed adjacent to the writing tip        and configured to transmit an acoustic signal;    -   at least one receiving unit as discussed above; and    -   a processor, associated with the at least one receiving unit,        configured to process the received acoustic signal, for        determining presence of the digital pen in a predefined area,        and to trigger a predefined functionality upon the determining        presence; and    -   a map, configured to graphically map the predefined area, so as        to assist a user in positioning the digital pen in the        predefined area.

According to a third aspect of the present invention there is provided adigital pen system comprising:

-   -   a digital pen having an electric circuit, an acoustic        transmitter configured to transmit acoustic signals, detached        from the electric circuit, and a resilient holder, configured to        press the electric circuit into contact with the transmitter        upon applying a mechanical pressure to the resilient holder, so        as to electrically connect the electric circuit and the        transmitter;    -   at least one receiving unit as discussed above; and    -   a processor, associated with the at least one receiving unit,        configured to process the received acoustic signals for        determining location of the digital pen.

According to a fourth aspect of the present invention there is provideda digital pen system comprising:

-   -   a digital pen having an acoustic transmitter, configured to        transmit acoustic signals and a switch assembly having two        switching points for pressing the assembly to achieve first and        second switching modes respectively, the assembly further having        a third mode selectable upon the two switching points being        pressed substantially simultaneously    -   at least one receiving unit as discussed above; and    -   a processor, associated with the at least one receiving unit,        configured to process the received acoustic signals for        determining location of the digital pen.

In an embodiment, the processor is further configured to detect a modeof the digital pen.

According to a sixth aspect of the present invention there is provided adigital pen system comprising:

-   -   a digital pen having an acoustic transmitter, configured to        transmit acoustic signals, and a smooth contact switch        configured to actuate the digital pen upon applying a pressure        on the smooth contact switch;    -   at least one of the receiving unit of claim 1; and    -   a processor, associated with the at least one receiving unit,        configured to process the received acoustic signals, for        determining location of the digital pen, the smooth contact        switch comprises a resilient element, mounted on a first side of        an open electric circuit and disconnected from a second side of        the electric circuit, the resilient element being compressible        into a position where the resilient element contacts the second        side of the electric circuit, thereby closing the electric        circuit, upon applying a writing pressure compressing the        resilient element into the position.

In an embodiment, the processor is further configured to detect theactuation of the digital pen.

According to a seventh aspect of the present invention there is provideda digital pen system comprising:

-   -   a digital pen having an acoustic transmitter, configured to        transmit acoustic signals;    -   at least one receiving unit according to claim 1; and    -   a processor, associated with the at least one receiving unit,        configured to process the ultrasound signals, for extracting        location of the digital pen, the extraction comprises        referencing a reference model comprising data pertaining to        expected reference signals.

An embodiment may comprise at least one attachment device, for removablyattaching the receiving unit to another item.

According to an eighth aspect of the present invention there is provideda digital pen system comprising:

-   -   a digital pen having an acoustic transmitter, configured to        transmit acoustic signals;    -   at least one receiving unit as discussed above, the receiving        unit further comprising a housing, and wherein the two acoustic        signal receivers are positioned inside the housing, less than 60        mm apart from each other, and configured to receive an acoustic        signal from the digital pen; and    -   a processor, associated with the at least one receiving unit,        configured to process the acoustic signal, for determining        location of the digital pen.

In an embodiment, the housing comprises a changeable cover element.

According to a ninth aspect there is provided a receiving unit forreceiving an acoustic signal from a digital pen, comprising:

at least two ultrasound receivers, for receiving ultrasound signals fromthe digital pen;

an electric circuit connected to said ultrasound receivers, andconfigured to extract ultrasound signals received by said ultrasoundreceivers, to enable tracking a position of said pen,

a biased clip for holding paper in fixed relationship to said ultrasoundreceivers, the biased clip comprising a metal clip, a depression foraccurate location of said paper and wherein said depression is foraccurate location of a corner of said paper.

According to a tenth aspect there is provided a receiving unit forreceiving an acoustic signal from a digital pen, comprising:

at least two ultrasound receivers, for receiving ultrasound signals fromthe digital pen;

an electric circuit connected to said ultrasound receivers, andconfigured to extract ultrasound signals received by said ultrasoundreceivers, to enable tracking a position of said pen,

a biased clip for holding paper in fixed relationship to said ultrasoundreceivers, the biased clip comprising a metal clip, a depression foraccurate location of said paper and wherein said depression is foraccurate location of a middle of a side of said paper.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. The materials, methods, andexamples provided herein are illustrative only and not intended to belimiting.

Implementation of the method and system of the present inventioninvolves performing or completing certain selected tasks or stepsmanually, automatically, or a combination thereof. Moreover, accordingto actual instrumentation and equipment of preferred embodiments of themethod and system of the present invention, several selected steps couldbe implemented by hardware or by software on any operating system of anyfirmware or a combination thereof. For example, as hardware, selectedsteps of the invention could be implemented as a chip or a circuit. Assoftware, selected steps of the invention could be implemented as aplurality of software instructions being executed by a computer usingany suitable operating system. In any case, selected steps of the methodand system of the invention could be described as being performed by adata processor, such as a computing platform for executing a pluralityof instructions.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin order to provide what is believed to be the most useful and readilyunderstood description of the principles and conceptual aspects of theinvention. In this regard, no attempt is made to show structural detailsof the invention in more detail than is necessary for a fundamentalunderstanding of the invention, the description taken with the drawingsmaking apparent to those skilled in the art how the several forms of theinvention may be embodied in practice.

In the drawings:

FIG. 1 a simplified block diagram illustrating a digital pen, accordingto a preferred embodiment of the present invention.

FIGS. 2 a and 2 b are exemplary depictions of a resilient holder,deployed inside a digital pen, according to a preferred embodiment ofthe present invention.

FIG. 3 is an exemplary depiction of a digital pen having a switchassembly comprising two switches, according to a preferred embodiment ofthe present invention.

FIG. 4 a is a simplified block diagram schematically illustrating anexemplary switch assembly mechanical design, according to a preferredembodiment of the present invention.

FIG. 4 b is a diagram showing a first exemplary cover element for aswitch assembly, according to a preferred embodiment of the presentinvention.

FIG. 4 c is a diagram showing a second exemplary cover element for aswitch assembly, according to a preferred embodiment of the presentinvention.

FIGS. 5 a-1 and 5 a-2 are simplified block diagrams depicting a touchswitch, according to a preferred embodiment of the present invention.

FIG. 5 b is a simplified diagram, illustrating an adhesive having avent, according to a preferred embodiment of the present invention.

FIG. 6 a is a simplified diagram illustrating a first digital pen havinga changeable cover element, according to a preferred embodiment of thepresent invention.

FIG. 6 b is a simplified diagram illustrating a second digital penhaving a changeable cover element, according to a preferred embodimentof the present invention.

FIG. 6 c is a simplified diagram illustrating a third digital pen havinga changeable cover element, according to a preferred embodiment of thepresent invention.

FIG. 6 d is a simplified diagram illustrating a fourth digital penhaving a changeable cover element, according to a preferred embodimentof the present invention.

FIG. 7 a is a simplified block diagram illustrating a first retractabledigital pen according to a preferred embodiment of the presentinvention.

FIG. 7 b is a simplified diagram illustrating a second retractabledigital pen according to a preferred embodiment of the presentinvention.

FIG. 8 a is a simplified block diagram illustrating a second retractabledigital pen, according to a preferred embodiment of the presentinvention.

FIG. 8 b is a simplified diagram illustrating a second retractabledigital pen, according to a preferred embodiment of the presentinvention.

FIG. 9 is a simplified block diagram schematically illustrating adigital pen having two acoustic transmitters according to a preferredembodiment of the present invention.

FIG. 10 is a diagram schematically illustrating a digital sleeve for awriting instrument, according to a preferred embodiment of the presentinvention.

FIG. 11 a-11 e are schematic depictions of a digital pen's grating for awriting instrument, according to a preferred embodiment of the presentinvention.

FIG. 12 is a schematic depiction of a first receiving unit for receivingan acoustic signal from a digital pen, according to a preferredembodiment of the present invention.

FIG. 13 is a schematic depiction of a second receiving unit forreceiving an acoustic signal from a digital pen, according to apreferred embodiment of the present invention.

FIG. 14 is a simplified block diagram illustrating a digital pen system,according to a preferred embodiment of the present invention.

FIG. 15 is a simplified block diagram illustrating a decoding unit,according to a preferred embodiment of the present invention.

FIG. 16 is a simplified block diagram illustrating exemplary componentsof a mathematical model for incorporating into a maximum likelihooddetector, according to a preferred embodiment of the present invention.

FIG. 17 is a two-part graph showing an exemplary correlation function,according to a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present embodiments comprise a digital pen, a digital sleeve, areceiving unit, and a digital pen system.

The principles and operation of a digital pen, a digital sleeve, areceiving unit, and a digital pen system according to the presentinvention may be better understood with reference to the drawings andaccompanying description.

The present invention attempts to overcome drawbacks of traditionaltechnologies, some of which are described hereinabove in the backgroundand field of invention section. The present invention attempts toimprove current technologies by introducing and implementing new ideasinto the design of a viable product, be it a digital pen, a digitalsleeve, a receiver for acoustic signals transmitted from a digital pen,or a digital pen system.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

Reference is now made to FIG. 1, which is a simplified block diagramillustrating a digital pen, according to a preferred embodiment of thepresent invention.

A digital pen 1000, according to a preferred embodiment of the presentinvention includes at least one acoustic transmitter 100, preferablydeployed adjacent to the pen's tip, and an electric circuit 110 such asa flexible printed electric circuit board (PCB) 120 which includes aconnection to an electric power source, such as a miniature battery.

Preferably, the acoustic transmitter 100 is an ultrasound transducer.

Optionally, the ultrasound transducer is a piezoelectric transducerwhich converts electrical energy into ultrasound signals. Piezoelectriccrystals have the property of changing size when an electric voltage isapplied to them. By applying an alternating electric voltage (AC) on apiezoelectric crystal, the crystal is caused to oscillate at very highfrequencies producing ultrasound signals comprised of very highfrequency sound waves.

Preferably, the ultrasound transducer is made of Polyvinylidene Fluoride(PVDF), which is flexible plastic polymer, bearing piezoelectricproperties.

The acoustic transmitter 100 is electrically connected to the circuit110, which may be detached from the transmitter 110, say to allowassembling an ink refill inside in the pen.

However, the acoustic transmitter 100 is too sensitive to allow heatingfor soldering, for electrically connecting the transmitter 100 to theelectric circuit 110, or even the attachment of plastic to thetransmitter 100. Though screwing the transmitter is optional, it is notsuitable for fast high volume production.

A preferred embodiment of the present invention attempts to overcome thedifficulty in electrically connecting the transmitter 100 and theelectric circuit 110, using a resilient holder 120.

The resilient holder 120 presses the electric circuit 110 into contactwith the acoustic transmitter 100, upon applying a mechanical pressureon the resilient holder 120, so as to electrically connect the electriccircuit 110 and the acoustic transmitter 100.

Optionally, the resilient holder 120 is further configured to push theflexible PCB into a position, in order to allow the placement ofcomponents such as IR transmitters in certain positions.

Preferably, the resilient holder 120 facilitates the digital pen's 1000being smaller than known in the art digital pens.

Reference is now made to FIG. 2 a which is an exemplary depiction of aresilient holder deployed inside a digital pen, according to a preferredembodiment of the present invention.

A digital pen 2000 according to a preferred embodiment of the presentinvention has an elongated body terminating in a writing tip, a writingelement 220 protruding from the writing tip, an acoustic transmitter 210deployed adjacent to the writing tip, and an electric circuit 240 suchas a flexible PCB 240 (flexible printed circuit board) or conductors.

The digital pen's writing element may be, but is not limited to an inkrefill, a pencil tip, a marker, etc. The digital pen may also include aneraser. The digital pen may also allow a user to change the color ofwriting, say using the switch assembly, described in detail hereinbelow. Optionally, the writing element is rather a sharpened tip whichdoes not physically write.

Preferably, the digital pen 2000 further includes a resilient holder 250pressing the flex PCB 240 (or the conductors) into contact with theacoustic transmitter 210, for electrically connecting the flex PCB 240and the acoustic transmitter 210 (and to the pen body).

Preferably, the resilient holder 250 may be made of conductive material,in order to increase electrical conductivity between the Flex PCB 240and the acoustic transmitter 210.

Optionally, the electric conductivity between the Flex PCB 240 and theacoustic transmitter 210 may be increased by deploying gold contacts onthe resilient holder 250.

The resilient holder 250 secures electrical contact between the Flex PCB240 and the acoustic transmitter 210. Optionally, as a result ofmechanical pressure applied on the resilient holder 250, say from thepen's housing.

In a preferred embodiment, the resilient holder 250 is in the shape of a“U”: solid above and open below, such that the pen body is kept tight bythe bottom part of the resilient holder 250. The “U” shaped resilientholder 250 comprises a base and extensions arising perpendicularly fromthe base, and is configured for location of the flex PCB 240 or anyother first electric circuit, and a second electric circuit, within theconfines of a housing. The resilient holder 250 brings about an electriccontact between the two electric circuit, due the confinement within thehousing, thereby connecting the two circuits. By connecting the twocircuits, the resilient holder 250 electrically connects the flexiblePCB 240 and the acoustic transmitter 210.

Preferably, the resilient holder 250 has some elasticity so as to enableeasy assembly, by putting all parts and sliding the resilient holder 250to position. The mechanical force is kept by elastic lugs on the upperpart consisting of the base hereinabove. The elastic lugs push theholder up (while the bottom part is secured to the pen's body asexplained hereinabove), as explained hereinabove.

Reference is now made to FIG. 2 b which shows an exemplary a digital penhaving a resilient holder, according to a preferred embodiment of thepresent invention.

A digital pen 2000 has a resilient holder 290, as described hereinabove.The resilient holder 290 further has leaf springs 292. The leaf springs292 are configured to apply pressure on an acoustic transmitter'sribbon, thereby connecting the acoustic transmitter to the flexible PCB295, as described hereinabove.

Reference is now made to FIG. 3 which an exemplary depiction of adigital pen having a switch assembly comprising two switches, accordingto a preferred embodiment of the present invention.

A digital pen 3000, according to a preferred embodiment of the presentinvention includes at least one acoustic transmitter 310, preferably anultrasound transducer.

The digital pen 3000 further comprises a switch assembly 320 having atleast two switches. The digital pen 3000 has a certain mode which a usermay select by pressing at least two of the switches substantiallysimultaneously.

Optionally, the switch assembly 320 is mechanically designed, accordingto known in the art techniques, with a position associated with thecertain mode of the pen. Preferably, the position is accessible onlywhen the user presses the two switches of the switch assembly 320simultaneously, or almost simultaneously.

Reference is now made to FIG. 4 a, which is a block diagramschematically illustrating an exemplary switch assembly mechanicaldesign, according to a preferred embodiment of the present invention.

A switch assembly, according to a preferred embodiment of the presentinvention, has two switches 410,420, mounted on a switching rod 450. Theswitching rod is balanced about a fulcrum 470. Preferably, the fulcrum470 is urged up by a spring.

A user may push the first switch 410, thus putting the assembly in afirst position (a), or push the second switch 420, thus putting theassembly in a second position (b), or toggle between the two positions(a,b).

When a user pushes the two switches 410,420, at the same time, or almostat the same time, the pressure applied on the two switches 410,420simultaneously pushes the fulcrum 470 against the spring, and puts theassembly in a third position (c) where both switches are pressed. Thethird position may be associated with a certain mode, as describedhereinabove.

Optionally, the switch assembly 320 is electrically designed, accordingto known in the art techniques, with a position associated with thecertain mode of the pen. The position is accessible only when the userpresses the two switches of the switch assembly 320 substantiallysimultaneously.

Reference is now made to FIG. 4 b, which a block diagram showing a firstexemplary cover element for a switch assembly, according to a preferredembodiment of the present invention.

Switch assembly 320, may further have cover element 4000. The coverelement 4000 has right and left protrusions 4100 (or regressions),guiding a user press one of the two switches 410,420, as explained ingreater detail hereinabove. The cover element 4000 further has a centralprotrusion 4200 (or regression), guiding the user, to apply a pressuresubstantially simultaneously on the two switches 410,420, thus bringingthe switch assembly into the third position, described hereinabove.

Reference is now made to FIG. 4 c, which is a block diagram showing asecond exemplary cover element for a switch assembly, according to apreferred embodiment of the present invention.

Cover element 4500 is mount on the switch assembly, such that theprotrusions (regressions) are positioned above their two switches 4520,described hereinabove.

Reference is now made to FIG. 5 a, which is a simplified block diagramdepicting a touch switch, according to a preferred embodiment of thepresent invention.

A digital pen according to a referred embodiment of the presentinvention includes a smooth contact switch, configured to actuate thedigital pen upon applying a mechanical pressure on the smooth contactswitch.

The smooth touch switch is assembled inside the digital pen, such thatthe mechanical pressure is applied on the smooth contact switch when thepen touches a surface, such as a sheet of paper, say when a user writesusing the digital pen.

Preferably, the applied pressure may be very small, preferably less thantwenty five grams. More preferably, the switch activation traveldistance is very small (say, less than 0.1 mm), and not sensed by theuser who uses the pen.

The smooth touch switch may be mounted on an open electric circuit, suchas a flexible printed circuit board (PCB), a regular circuit, or on twodetached circles of conductive material, etc As a result, no wires orribbons are needed to connect the switch to the electrical circuit.

In a preferred embodiment, a concentric adhesive 510 with electricalconduction properties is applied on a resilient element 530,electrically connected to one side of an open electric circuit.

The upper part of the smooth touch switch is the flat and compressibleresilient element 530 having conducting properties, mounted on theadhesive 510, as shown in a cross sectional view (5 a-2) along the AAline of the bottom view (5 a-1). Optionally, the resilient element ismade of a conductive material or the conductive properties are given tothe resilient element 530, by a adding an additive, such as a conductiveink or glue to the resilient element 530.

A pressure in the center of the upper part of the resilient element 530of the smooth touch switch compresses the resilient element 530 into aposition forming an electric path from a second side of the openelectric circuit, through the resilient element 530, through theconcentric adhesive 510, and to the first side of the electric circuit,thus closing the electric circuit, thereby actuating the digital pen.

The actuating pressure is controlled by the thickness of the concentricadhesive 510, inner diameter of the concentric adhesive 510 andthickness of the resilient element 530.

Optionally, the resilient element is made of PolyethyleneTerepthalate—(PET) material covered with conductive ink, and theconcentric adhesive is a very thin layer, of no more then 0.1 mm, andmade of 3M™ Z-Axis, or similar products. Optionally the resilientelement 530 is made of conductive metal.

Reference is now made to FIG. 5 b which is simplified block diagram,illustrating an adhesive having a vent, according to a preferredembodiment of the present invention.

Preferably, the adhesive 5100 used in the smooth touch switch, describedhereinabove, includes vent holes 5150, for relieving air pressuretrapped inside the cavity formed by the resilient element 530, theadhesive, and the electric circuit closed by the smooth touch switchupon compressing the resilient element 530.

Reference is now made to FIG. 6 a, which is a simplified diagramillustrating a first digital pen having a changeable cover element,according to a preferred embodiment of the present invention.

Preferably, a digital pen according a preferred embodiment of thepresent invention, has an inner structure which holds the functionalparts together and a housing having a changeable cover element (skin).

The inner part may hold an acoustical transducer, IR emitters, anelectric circuit such as a flexible PCB, switches, etc. The housingcovers the inner part and has some mechanical interfaces which allow itsconnection to the inner part. The housing may have additional functionalproperties, such as a battery holder.

In a preferred embodiment, there is introduced a variety of colorful andfashioned changeable cover elements, thus providing a range of covers(skins) for the digital pen.

Optionally, a manufacturer of the pen assembles the pen with one coverelement of a variety of cover elements and the end user does not changethe cover element. Preferably, an end-user is allowed to change thecover element of the housing, thus giving the digital pen differentappearance and different feel or texture

For example, a digital pen 6100 has a housing which includes a centralchangeable cover element (skin) 610, connected to battery supportchassis 611 on one side, and to a pen tip 612, on the other side.Optionally, the central changeable cover element (skin) 610 is connectedto the battery support chassis 611 and the pen tip 612, utilizing snaplocks 615, visible or hidden, as known in the art.

Reference is now made to FIG. 6 b, which is a simplified diagramillustrating a second digital pen having a changeable cover element,according to a preferred embodiment of the present invention.

a digital pen 6200 has a housing which includes a changeable coverelement (skin) 6210, connected to a pen tip 6220.

Reference is now made to FIG. 6 c, which is a simplified diagramillustrating a third digital pen having a changeable cover element,according to a preferred embodiment of the present invention.

a digital pen 6300 has a housing which includes a changeable coverelement (skin) 6310, connected to a battery cover 6320.

Reference is now made to FIG. 6 d, which is a simplified diagramillustrating a fourth digital pen having a changeable cover element,according to a preferred embodiment of the present invention.

a digital pen 6400 has a housing which includes an upper changeablecover element (skin) 6410, connected to a lower cover 6420.

Preferably, the digital pen is a retractable digital pen allowingcovering the writing element at the tip of the pen, say at the tip of anink cartridge deployed inside the pen, (or stylus, or pencil).

Reference is now made to FIG. 7 a which a simplified block diagramillustrating a first retractable digital pen according to a preferredembodiment of the present invention.

According to a preferred embodiment, the digital pen 700 has a rotatingpart 710 which moves forward or backwards when a user rotates the part710.

The rotating part 710 moves forward and covers a writing element 720,protruding from the tip 715 of the digital pen 700, when the userrotates the part 710 in one direction. The rotating part 710 movesbackwards, and exposes the writing element 720, as the user rotates thepart 710 in an opposite direction.

Optionally, the rotational movement of the rotating part 710 istransformed into a linear movement where the rotating part 710 movesforward, for covering the writing element 720, or backwards, forexposing the writing element 720. The transformation may be facilitatedby a helical track, guiding the rotating part 710, as known in the art.

Reference is now made to FIG. 7 b which a simplified diagramillustrating a second retractable digital pen according to a preferredembodiment of the present invention.

A digital pen's housing includes a skin 7120 and a retractable tip 7110.The retractable tip 7110 is connected by a spiral mechanism 7100 to theskin 7120. The spiral mechanism 7100 causes a linear movement of the tipin and out. The rotation motion is applied by the user between the tip7110 and the skin 7120.

Reference is now made to FIG. 8 a which a simplified block diagramillustrating a third retractable digital pen, according to a preferredembodiment of the present invention.

A retractable digital pen 800 comprises an elongated housing 805,covering an elongated body 820 terminating in a writing tip, wherefrom awriting element 810, such as a tip of an ink refill protrudes.

The elongated body 820 may be moved forward, to expose the writingelement 810, and backwards to cover the writing element inside thehousing 805 of the digital pen 800.

Optionally, the elongated body 820 is urged backwards by a spring 830,thus pushing the elongated body 820 into a position where the writingelement 810 is covered by the housing.

Preferably, the elongated body 820 is securable into a position wherethe writing element 810 is exposed, by a securing means 850. Optionally,there may be used a snap, a lock, etc, for locking the elongated body onthe edge of the housing 805.

Optionally, a digital pen, according to a preferred embodiment may havemoving parts, such a refill, a skeleton, a tip, a battery house, anyother part, or a combination thereof. The movement between the movingparts may be facilitated utilizing designs similar to the designeddescribed above, using FIG. 7-8.

According to a preferred embodiment of the present invention, there areput several infrared (IR) emitters, on several points of the digital penfor more robustness.

As a result, if one of the IR emitters is covered, say by the hand of auser while holding the digital pen, the other parts maintain the linkwith a receiver.

Examples of the possible points on the digital pen where the IR emittersmay be deployed include, but are not limited to: the bottom part of thedigital pen, the upper part, on the top of the pen, on a flexible PCBinstalled in the digital pen (as described hereinabove), etc.

Preferably, the housing of the digital pen includes a soft material suchas rubber, so as to provide better convenience for a user holding thedigital pen.

Reference is now made to FIG. 8 b which a simplified block diagramillustrating a fourth retractable digital pen, according to a preferredembodiment of the present invention

A digital pen may have a retractable skeleton 7250, pushed by a button7270 mounted on top of the digital pen, utilizing and a lockingmechanism 7200.

Reference is now made to FIG. 9 which is a block diagram schematicallyillustrating a digital pen having two acoustic transmitters according toa preferred embodiment of the present invention.

A digital pen 900 may have two acoustic transmitters 930. Installing twoacoustic transmitters in a digital pen may have several advantages,which may include, but are not limited to the following:

-   1) Allowing the receiver to estimate the five dimensional (5D)    location of the pen which includes the three dimensional location,    and leaning angles of the digital pen, or a six dimensional (6D)    location of the pen, which includes the five directional (5D)    location as well data relating to rotation of the digital pen.-   2) Estimating more accurately the writing element's position and    compensating for the distance difference between the transducer and    position of the writing element.-   3) Allows gaming functions, using the digital pen as a joystick.

Reference is now made to FIG. 10 which is a diagram schematicallyillustrating a digital sleeve for a writing instrument, according to apreferred embodiment of the present invention.

The digital sleeve 1000 comprises an acoustic signal transmitter, fortransmitting an acoustic signal. The digital sleeve 1000 may alsocomprise an electric circuit, a power source, or other elements, asdescribed for a digital pen hereinabove.

A digital sleeve 1000, according to a preferred embodiment, may bemounted on a regular writing instrument 1100, such as a pen, a pencil, amarker, etc.

According to a preferred embodiment of the present invention, thedigital sleeve 1000 may be worn on a finger. For example, EposTechnologies™ provides a stylus-at-your-fingertip product.

Preferably, the digital sleeve 10000 further includes a writing sensor10200. The writing sensor 10200 is configured for detecting a movement(or a friction) of the writing device 10100, relative to the digitalsleeve 10000 mounted thereon.

That is to say, as a user, holding a pen mounted with the sleeve 1000,starts writing with the pen, a relative movement (or friction) occursbetween the pen touching a paper and the sleeve 1000. The relativemovement (or friction) is sensed by the writing sensor 10200. Thewriting sensor 10200 in turn, actuates the acoustic transmitter, throughelectric circuitry. Then, the acoustic transmitter transmits theacoustic signals, say to a receiving unit, as described in greaterdetail for a digital pen system herein below.

Reference is now made to FIG. 11 a-e, which are schematic depictions ofa digital pen's grating for a writing instrument, according to apreferred embodiment of the present invention.

Typically, an acoustic transmitter, specifically—an ultra soundtransducer has some irregularities. The irregularities make thetransducer not entirely omni directional.

The irregularities result from a part of the transducer having aninherent defect, because the transducer is made from a rectangular foillaminated to form a cylinder. The lamination forms a passive part whichdoes not radiate acoustic energy. The inherent defect causes the signalin front of the defect to be much weaker than in front of other parts ofthe ultrasound transducer.

Typically, the position of the digital pen is determined utilizing analgorithm, based on a measurement of TOA (time of arrival) of theacoustic signals from the acoustic transmitter. Usually the algorithmcompares the TOA of the signals with IR signals transmitted from thedigital pen.

As a result of the inherent defect, the sum of the signals received at agiven point in the space surrounding the acoustic transmitter has aphase shift, in comparison to other points at a similar distance awayfrom the acoustic transmitter.

A digital pen, according to a preferred embodiment, includes an acousticwave guide, positioned adjacent to an acoustic transmitter of thedigital pen.

Preferably, the acoustic wave guide comprises a plurality of fins 1110radiating outwardly in a direction away from the acoustic signaltransmitter.

More preferably, the fins 1110 are positioned so as to spatially dividethe space surrounding the acoustic transmitter into directional sectors.

The fins 1110 substantially isolate acoustic signals transmitted by theacoustic transmitter through one of the sectors from acoustic signalstransmitted from the acoustic transmitter through the other sectors.

That is to say, to eliminate the shift in location, the fins 1110 arepositioned so to as to divide the space around the acoustic transmitterinto sectors, such that each sector is decoupled or isolated from theother sectors.

As a result of the division of space around the acoustic transmitterinto significantly isolated sectors, the phase shift is significantlyeliminated. The elimination of phase shift may improve the results ofacoustic signal correlation based position decoding techniques. However,the amplitude of the sum of signals transmitted through each point inone of the sectors around the acoustic transmitter is reduced, assignals from the other sectors are significantly eliminated from thesector.

Optionally, the grating around the acoustic transmitter may be designeddifferently than the above described fin design.

For example, the grating may comprise a spiral opening keeping a singleopening, a grating coming upwards combined with a grating comingdownwards (keeping an opening to free air in between), etc.

According to a preferred embodiment of the present invention, there isprovided a receiver configured to receive acoustic signals transmittedfrom a digital pen, to be used for determining location of the digitalpen, say for automatically digitizing hand writing carried out using thedigital pen.

Reference is now made to FIG. 12 which is a schematic depiction of afirst receiving unit for receiving an acoustic signal from a digitalpen, according to a preferred embodiment of the present invention.

A receiving unit 1200, configured to receive acoustic signals from adigital pen may have a metal plate 1210 mounted on the body 1220 of thereceiving unit, for securing the receiving unit 1200 to a sheet ofpaper.

Pressing the metal plate one end 1210-a makes the other end 1210-b opena gap between the other end 1210-b and the body 1220 of the receivingunit 1200. Through the opened gap, a sheet of paper may be insertedbetween the plate's end 1210-b and the body 1220 of the receiving unit1200.

Releasing the metal plate pressed end 1210-a makes the other end 1210-bget back to its natural position and embed a force on the paper sheetwhich is pressed between the plate's end 1210-b and the body 1220 of thereceiving unit 1210.

The metal plate 1210 and the body 1220 of the receiving unit 1200 mayhave additional non-flat surface properties (such as rubber pads) whichallow more friction between the paper and the receiving unit's body1220.

Preferably, the metal plate 1210 may be shaped, so as to cause a slightdeformation of the paper, in order to have a better grip of the papersheet.

One or more receiving unit(s) 1200 may be fit on the paper sheet'scenter, or on the sheet's edges.

Preferably, the receiving unit body 1200 and plate have stoppers 1212that fit the 90 degrees of a paper sheet's corner (and hold thereceiving unit at 45 degrees).

The placement of the receiving unit 1200 on the corner of the papersheet instead on the sheet's middle has several benefits, such as:Repeatability, Accuracy—a receiving unit placed on the corner has abetter perspective, improving its accuracy, Less dead zones—as theoperating angle of a receiving unit placed at the corner of the papersheet is much smaller than when a receiving unit is place in the middleof the paper.

Reference is now made to FIG. 13 which is a schematic depiction of asecond receiving unit for receiving an acoustic signal from a digitalpen, according to a preferred embodiment of the present invention.

A receiving unit 1300, according to a preferred embodiment of thepresent invention includes two microphones 1330.

Optionally, the two microphones are ultrasound receivers, as known inthe art.

Preferably, the two microphones are electret microphones oralternatively MEMS microphones. Electret microphones are miniaturemicrophones that work on condenser microphone principles, as known inthe art, but have permanently charged polymer diaphragms. Electretmicrophones have miniature preamplifiers built in, and require lowvoltage direct current (DC) power (typically from a 1.5 to 18 voltsbattery).

Electret microphones are widely used in hand held devices—such as mobilecomputer games, mobile phones, etc.

The receiving unit 1300 further includes an electric circuit.

The electric circuit is configured to extract ultrasound signal,received by the microphones 1330, say by implementing frequency downconversion, signal filtration, signal amplification techniques, or othermethods.

Some of the methods used by the electric circuit are described ingreater detail in the applicant's International Application No.PCT/IL03/00309, entitled “Method and system for obtaining positionaldata”, filed on Apr. 14, 2003.

According to a preferred embodiment of the present invention, the twomicrophones 1330 are positioned in a distance of less then 65 mm fromeach other.

The signals received form the two microphones 1330 positioned less than65 mm away from one another, may be processed for generating positionaldata relating to the digital pen.

The processing may be carried out using decoding methods, say utilizingmodels of the transmitted and received signals as described in graterdetail herein below.

According to a preferred embodiment of the present invention, aprocessor, connected with one or more receiving unit(s), is configuredto process acoustic signal, received at the receiving unit(s), fordetermining presence of the digital pen in a predefined area.

Preferably, the processor may be configured to trigger a predefinedfunctionality when a user places the digital pen in a predefined area.

Optionally, the user may be provided a printed map or menus, andposition the receiving unit(s) on the map or menus. When the userpositions the digital pen on an icon, representing the predefined area,printed on the paper, the digital pen is present in the predefined area.Consequently, the predefined functionality is triggered by theprocessor.

For example, the user may be provided a printed menu having drawn iconssuch as an eraser, a marker, etc. The user may deploy the receivingunit(s) on the printed menus. If the user places the digital pen on theeraser icon, the processor switches into an erasing mode and the digitalpen functions as an eraser. If the user places the digital pen on themarker icon, the processor switches into a marker mode and the digitalpen functions as a marker.

Preferably, the housing of 1320 of the receiving unit 1300 is used as anassembly jig. A worker assembling the receiving unit 1300 may insert themicrophones 1330 into their position inside the body 1320, and solder aprinted electric circuit board (PCB) into a position inside the body1320. The worker may then connect the PCB to the microphones 1330.

Optionally, the receiving unit 1300 may be removable attached to anotheritem, such as a paper clipboard used by a student, etc.

Preferably, the housing 1320 of the receiving unit 1300 includes achangeable cover element.

The changeable element may provide a user of the receiving unit 1300, amanufacturer of the receiving unit 1300, or both, with the option tochange the color and appearance of the receiving unit 1300.

Optionally, the housing 1320 of the receiving unit 1300 may also house aserial interface cable, rolled in and out from the housing. Preferably,a connector at the end of the interface cable may be clipped to thehousing 1320. The housed interface cable helps to keep the receivingcompact.

Reference is now made to FIG. 14 which is a simplified block diagramillustrating a digital pen system, according to a preferred embodimentof the present invention.

A digital pen system 1400 includes a digital pen 1410, and one or moredigital pen receiver(s) 1420, as described in greater detailhereinabove.

The system 1400 further includes a processor 1450, communicating withthe receiving units(s) 1420.

The processor 1450 is configured to process acoustic signals,transmitted from the digital pen 1410 and received by the receivingunit(s) 1420.

Through the processing of the received acoustic signals, the processor1420 determines the location of the digital pen 1410.

Optionally, the processing further includes determining the presence ofthe digital pen 1410 in a predefined area, and triggering a predefinedfunctionality upon the determined presence in the predefined area, asdescribed hereinabove.

According to a preferred embodiment of the present invention, thelocation of the digital pen according to the acoustic signalstransmitted from the digital pen is carried out utilizing a decodingalgorithm. The decoding algorithm may be implemented in a decoding unit1470. The decoding unit 1470 may be implemented as a part of theprocessor 1450, as a part of a device communicating with to theprocessor 1450, as a part of the receiving unit(s) 1420, etc.

Reference is now made to FIG. 15 which is a simplified block diagramillustrating a decoding unit, according to a preferred embodiment of thepresent invention.

A decoding unit 70 includes a maximum likelihood detector 72, which usesa channel mathematical signal model 77, a correlator 71, a maximumlikelihood detector, a path estimator and transmitter timing estimator.

The maximum likelihood detector 72 generates most likely distance data,relating to the distance of the digital pen from a receiving unit, basedon the acoustic signals received from the digital pen, and feeds thepath estimator 73 with the most likely distance data.

The maximum likelihood detector 72 estimates the transmitter positionand feeds the path estimator 73 with several options for location of thetransmitter, each option having a probability associated therewith. Thepath estimator 73 further uses previously calculated possible positionsfrom a sampling bank 75 (and their probabilities), provided by atransmitter timing estimator 76, in order to choose the right estimatedcoordinates 74 of the position of the transmitter

The decoding algorithm is used to convert digitized versions of thedigital pen's acoustic signals into position coordinates for passing toa local computer operating system, a computer application, or the like.

The decoding algorithm preferably takes into account the relatively lowsampling frequency capabilities likely to be available, by carrying outfrequency down conversion. Preferably, the path estimator 73 uses knownin the art methods of interpolation, for compensating for the relativelylow sampling rate.

In addition, the algorithm preferably includes an ability to handlenoise.

The algorithm is preferably adapted for other specific issues, involvedin the handling of the acoustic signals transmitted from the digitalpen.

Traditional position location methods concentrate on the use of veryshort and energetic acoustic signals, as the location signal. In orderto achieve good resolution, the traditional methods dictate highsampling frequencies, typically higher than 400 KHz, in order to be ableto find such short location signals and not miss them entirely.

By contrast, the present embodiments preferably do not use samplingrates higher than 44.1 KHz, since such frequencies are incompatible withthe installed base of sound processing equipment, such as the electretmicrophones.

Furthermore, it is recommended to keep the beacon signal sound frequencyhigher than 20 KHz, that is within the ultrasonic range, so that usersdo not hear it.

In another preferred embodiment of the invention, the sampling rate maybe higher than the 44.1 KHz, say 100 KHz. This is possible by areceiving unit which is configured for a high sampling rate. The highersampling rate enables better noise rejection of the audio band andhigher bandwidth of the transmitted signal.

A preferred embodiment of the present invention uses a solution in whichdata is modulated over an ultrasonic carrier signal or waveform. Thedata can be frequency modulated (FM), or phase modulated (PM), onto thecarrier comprising the ultrasonic signal. Optionally, other known methodmay be used.

The decoding algorithm preferably decodes the modulated signal andreconstructs the original position-information bearing signal from theresults of sampling thereof. In the present embodiment, it is preferredto use band-limited signals in order to achieve a desired resolutionlevel.

Preferably, continuous wave (CW) modulations such as spread spectrum andfrequency hopping are used in acoustic position finding, to overcomereverberation and multi-path effects.

A preferred embodiment of the present invention uses the maximumlikelihood detector 72, for decoding the signals received from thereceiving units, to determine the distances of the digital pen from theindividual receiving unit(s).

At the maximum likelihood detector 72, the acoustic signals receivedfrom the receiving units are compared to reference signals in a look-uptable (LUT) 68.

The comparison indicates a most likely signal, and from the most likelysignal, a distance is determined as the distance from which the signalwas most likely transmitted.

The maximum likelihood detector 72 preferably uses a full mathematicalsignal model 77 of the channel, against which to compare receivedsignals, so that a best match distance can be found.

As an alternative, the expected waveform can be sampled at the Nyquistrate, and any timing mismatch between the sampling points can beovercome by extrapolation functions, to reveal the distance.

Reference is now made to FIG. 16, which is a simplified block diagramillustrating exemplary components of a mathematical model forincorporating into a maximum likelihood detector, according to apreferred embodiment of the present invention.

The model 20 comprises an initial signal sequence S(t), generated in thesignal generator, which is fed into the transfer function of theacoustic transmitter 26 with its filter 25. The digital pen 14 isfollowed by the channel 27. The result is then fed to the reception pathin the receiver which includes transfer function 29 for the ultrasoundreceiver, and filtering 30.

The full modeling of the channel is useful in the design of the maximumlikelihood detector 72, in that it allows accurate expected signals tobe constructed against which the received acoustic signals, ideally,differ only in phase.

The detector (estimator) 70 is then relatively easily able todistinguish the most likely signal, which in turn corresponds to themost likely distance of the digital pen from the receiving unit.

Preferably, the infrared (IR) signal transmitted from the IRtransmitters, spread on the face of the digital pen, are used to set thestart of the delay, and also to synchronize clocks between the digitalpen and the receivers.

In FIG. 15, synchronization path 76 is also indicated on the model.

A skilled person will appreciate that acoustic signals have differingangular transfer functions. An equalizer may be used in order tocompensate for this fact.

The skilled person will appreciate that, instead of a model, a look-uptable may be used. Furthermore, other detectors may be used, and thereare several known decoders of FM signals, such as PLL (An electroniccircuit that consists of a phase detector, low pass filter andvoltage-controlled oscillator), I/Q demodulation, phase multiplicationetc.

Reference is briefly made to FIG. 17, which is a two-part graph showingan exemplary correlation function, according to a preferred embodimentof the present invention.

The top part 1710 of the graph shows the function, and the lower part1720 of the graph is an enlarged or zoomed view of the upper centralpart of the graph.

It is expected that during the life of this patent many relevant devicesand systems will be developed and the scope of the terms herein,particularly of the terms “Digital”, “Pen”, “Acoustic transmitter”,“Ultrasound transducer”, “Microphone”, and “Processor” is intended toinclude all such new technologies a priori.

Additional objects, advantages, and novel features of the presentinvention will become apparent to one ordinarily skilled in the art uponexamination of the following examples, which are not intended to belimiting. Additionally, each of the various embodiments and aspects ofthe present invention as delineated hereinabove and as claimed in theclaims section below finds experimental support in the followingexamples.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

1. A receiving unit for receiving an acoustic signal from a digital pen,comprising: at least two ultrasound receivers, for receiving ultrasoundsignals from the digital pen; an electric circuit connected to saidultrasound receivers, and configured to extract ultrasound signalsreceived by said ultrasound receivers, to enable tracking a position ofsaid pen, and a biased clip for holding paper in fixed relationship tosaid ultrasound receivers, the biased clip comprising a metal clip. 2.The receiving unit of claim 1, further comprising a depression foraccurate location of said paper.
 3. The receiving unit of claim 2,wherein said depression is for accurate location of one member of thegroup consisting of a corner of said paper and the middle of an edge ofsaid paper.
 4. The receiving unit of claim 1, wherein said clipcomprises a release-extension for opposing said bias to release saidclip.
 5. The receiving unit of claim 1, wherein said extractioncomprises referencing a reference model comprising data pertaining toexpected reference signals.
 6. The receiving unit of claim 1, furthercomprising friction pads located in association with said clip toimprove grip of said paper.
 7. The receiving unit of claim 1, wherein asurface of said receiving unit opposite said clip is shaped to causedeformation of the paper held by said clip to improve grip of saidpaper.
 8. The receiving unit of claim 1, further comprising elongatedgrips located to hold said paper at an oblique angle.
 9. The receivingunit of claim 1, wherein said ultrasound receivers are at least one ofthe group consisting of electret microphones and MEMS microphones. 10.The receiving unit of claim 1, further comprising at least oneattachment device, for removably attaching the receiving unit to anotheritem.
 11. The receiving unit of claim 1, wherein said at least twoultrasound receivers are positioned less than 65 mm apart from eachother.
 12. The receiving unit of claim 1, wherein said receiving unitfurther comprises a housing, configured to house said ultrasoundreceiver and having a changeable receiving unit cover element.
 13. Thereceiving unit of claim 12, being used with a digital pen comprising askeleton and a removable skin: the skeleton holding together in fixedrelationship: an electric circuit; an acoustic transmitter, detachedfrom said electric circuit, and configured to transmit acoustic signals;and the removable skin comprising a pen housing fitting over saidskeleton.
 14. The receiving unit of claim 13, wherein said pen housingand said receiving unit cover element correspond.
 15. A receiving unitfor receiving an acoustic signal from a digital pen, comprising: atleast two ultrasound receivers, for receiving ultrasound signals fromthe digital pen; an electric circuit connected to said ultrasoundreceivers, and configured to extract ultrasound signals received by saidultrasound receivers, to enable tracking a position of said pen, abiased clip for holding paper in fixed relationship to said ultrasoundreceivers, the biased clip comprising a metal clip, a depression foraccurate location of said paper and wherein said depression is foraccurate location of a corner of said paper.
 16. A receiving unit forreceiving an acoustic signal from a digital pen, comprising: at leasttwo ultrasound receivers, for receiving ultrasound signals from thedigital pen; an electric circuit connected to said ultrasound receivers,and configured to extract ultrasound signals received by said ultrasoundreceivers, to enable tracking a position of said pen, a biased clip forholding paper in fixed relationship to said ultrasound receivers, thebiased clip comprising a metal clip, a depression for accurate locationof said paper and wherein said depression is for accurate location of amiddle of a side of said paper.
 17. A digital pen system, comprising: adigital pen having an elongated body terminating in a writing tip, awriting element protruding from said writing tip, and an acoustic signaltransmitter deployed adjacent to said writing tip and configured totransmit an acoustic signal; at least one receiving unit according toclaim 1; a processor, associated with said at least one receiving unit,configured to process said received acoustic signal, for determiningpresence of said digital pen in a predefined area, and to trigger apredefined functionality upon said determining presence; and a map,configured to graphically map said predefined area, so as to assist auser in positioning the digital pen in said predefined area.
 18. Thedigital pen system of claim 17, wherein: the digital pen comprises anelectric circuit, an acoustic transmitter configured to transmitacoustic signals, detached from said electric circuit, and a resilientholder, configured to press said electric circuit into contact with saidtransmitter upon applying a mechanical pressure to said resilientholder, so as to electrically connect said electric circuit and saidtransmitter.
 19. A digital pen system of claim 17, wherein the digitalpen comprises an acoustic transmitter, configured to transmit acousticsignals and a switch assembly having two switching points for pressingsaid assembly to achieve first and second switching modes respectively,said assembly further having a third mode selectable upon said twoswitching points being pressed substantially simultaneously.
 20. Thedigital pen system of claim 19, wherein said processor is furtherconfigured to detect a mode of said digital pen.
 21. The digital pensystem of claim 17, wherein: said digital pen comprises an acoustictransmitter, configured to transmit acoustic signals, and a smoothcontact switch configured to actuate the digital pen upon applying apressure on said smooth contact switch; and said smooth contact switchcomprises a resilient element, mounted on a first side of an openelectric circuit and disconnected from a second side of said electriccircuit, said resilient element being compressible into a position wheresaid resilient element contacts said second side of said electriccircuit, thereby closing said electric circuit, upon applying a writingpressure compressing said resilient element into said position.
 22. Thedigital pen system of claim 21, wherein said processor is furtherconfigured to detect said actuation of the digital pen.
 23. The digitalpen system of claim 17, wherein: said digital pen comprises an acoustictransmitter, configured to transmit acoustic signals; and saidprocessor, is associated with said at least one receiving unit, andwherein said processing of said ultrasound signals, is for dynamicallyextracting a location of said digital pen, said extraction comprisingreferencing a reference model comprising data pertaining to expectedreference signals.
 24. The digital pen system of claim 23, furthercomprising at least one attachment device, for removably attaching saidreceiving unit to another item.
 25. The digital pen system of claim 17,wherein: said digital pen comprises an acoustic transmitter, configuredto transmit acoustic signals; the receiving unit further comprising ahousing, and wherein said two acoustic signal receivers are positionedinside said housing, less than 60 mm apart from each other, andconfigured to receive an acoustic signal from said digital pen; andwherein the processor is associated with said receiving unit, and isconfigured to process said acoustic signal, for determining location ofsaid digital pen.
 26. The digital pen system of claim 25, wherein saidhousing comprises a changeable cover element.